Hi all. My Mcintosh MC431M amp is acting funny lately (it's been working perfectly until couple weeks ago). Let me describe the problem in detail first:
After leaving my car off for several hours (like parking over night), if I turn on my audio system right after I start the car engine, then my MC431M will go into protect mode. It'd scream my speakers really loud for a second or so before it goes into protect. If I detach all the speakers from the amp before turning it on, the amp's both power meters (all four channels) will shoot over the scale, but it won't go into protect mode at all. I have to turn it off right away in order to prevent it from blowing off.
But the amp works fine if I just turn the key to on without starting the car engine. And if I let the amp stay on like this (keep playing music) for a few minutes then start the engine, then the amp will keep working even after the engine starts.
I measured the voltages feeding to the amp before and after the engine started. they were about 12.6V and 14.6V respectively, which seemed normal. I also tried swapping my 431M with my 420M. the 420M didn't give me any problem at all. So I believe my car's electrical system should be ruled out. I also checked all my speakers. There were nothing wrong with them either.
It first sounded like a capacitor problem to me. You know, after I turn on the 431M at low voltage (12.6V running on the battery only) for a while, all the capacitors would have been charged up, then it works at a higher voltage (14.6V after the engine starts). But if the amp is left off for a few hours, all the capacitors would have been discharged. When I supply 14.6V to the amp at this point, the problem occurs.
So I opened the amp and did find out a few leaking 50v 10uF capacitors (not too bad tho). Then I replaced these leaking caps. But to my surprise this didn't solve the problem. Now I'm really confused. All other parts look just fine, no burnt mark or anything. So what else could be the problem here? Can anyone help me please? Thanks!
After leaving my car off for several hours (like parking over night), if I turn on my audio system right after I start the car engine, then my MC431M will go into protect mode. It'd scream my speakers really loud for a second or so before it goes into protect. If I detach all the speakers from the amp before turning it on, the amp's both power meters (all four channels) will shoot over the scale, but it won't go into protect mode at all. I have to turn it off right away in order to prevent it from blowing off.
But the amp works fine if I just turn the key to on without starting the car engine. And if I let the amp stay on like this (keep playing music) for a few minutes then start the engine, then the amp will keep working even after the engine starts.
I measured the voltages feeding to the amp before and after the engine started. they were about 12.6V and 14.6V respectively, which seemed normal. I also tried swapping my 431M with my 420M. the 420M didn't give me any problem at all. So I believe my car's electrical system should be ruled out. I also checked all my speakers. There were nothing wrong with them either.
It first sounded like a capacitor problem to me. You know, after I turn on the 431M at low voltage (12.6V running on the battery only) for a while, all the capacitors would have been charged up, then it works at a higher voltage (14.6V after the engine starts). But if the amp is left off for a few hours, all the capacitors would have been discharged. When I supply 14.6V to the amp at this point, the problem occurs.
So I opened the amp and did find out a few leaking 50v 10uF capacitors (not too bad tho). Then I replaced these leaking caps. But to my surprise this didn't solve the problem. Now I'm really confused. All other parts look just fine, no burnt mark or anything. So what else could be the problem here? Can anyone help me please? Thanks!
Last I heard MAC still takes care of these very well if your the original owner and paid retail list for them. You might want to contact them and ask.
Bad caps are a common issue with these amps IMO. This and the protection circuitry and the soft clip circuitry seem to be linked and they can cause some really unique issues to arise. At least all of the above happened to mine anyway. I got mine second hand so MAC was not a option for repair or support.
One of these days I will pull mine out and give them the once over
I wish I could offer more but I have only repaired a dozen or so of these and they all seemed to have similar issues. Bad solder on the power devices, bad caps, and damaged input op-amps and malfunctioning Protection/ Soft clip circuitry. This is why I suggest MAC for service to you.
Bad caps are a common issue with these amps IMO. This and the protection circuitry and the soft clip circuitry seem to be linked and they can cause some really unique issues to arise. At least all of the above happened to mine anyway. I got mine second hand so MAC was not a option for repair or support.
One of these days I will pull mine out and give them the once over
I wish I could offer more but I have only repaired a dozen or so of these and they all seemed to have similar issues. Bad solder on the power devices, bad caps, and damaged input op-amps and malfunctioning Protection/ Soft clip circuitry. This is why I suggest MAC for service to you.Last I heard MAC still takes care of these very well if your the original owner and paid retail list for them. You might want to contact them and ask.
Bad caps are a common issue with these amps IMO. This and the protection circuitry and the soft clip circuitry seem to be linked and they can cause some really unique issues to arise. At least all of the above happened to mine anyway. I got mine second hand so MAC was not a option for repair or support.
One of these days I will pull mine out and give them the once over
Thanks a lot for your advice. But I also bought my 431M used. so going back to MAC is not gona be my option either
I'll take a closer look at the other caps again. As for the power devices, do you mean all the parts that are supplying the power? like all the transformer, transistors, resistors and caps? These are the ones that usually have bad/loose soldering? And for reparing the malfunctioning protection circuitry, you just need to replace the protection ic chip? Thanks!
Power devices are the transistors outputs and the Mosfet power supply semiconductor devices that are sink clamped and the leads seem to break there solder joints.
Be careful the clamps are none to sturdy either. I never broke any of them but bending them can be easily done and then they have inadequate pressure to actually clamp the semiconductor to the sink properly. The sink is a monster and I loved that part of their design.
As for the protection circuitry well it uses direct feedback < I.E. non isolated > to the supply to soft clip the amp and here in lies the problem. If and when it does act up it causes the amp supply to do very odd things. Without a schematic bypassing is very troublesome and this is the only way I know to get the amp to turn on and be tested. Sort of a catch 22. MAC never was very open about this amp and of course PDF file sharing is something I never knew about from them even now..
The amp is really nothing special. it is based on Sanyo Fisher designs and has Sanyo channel driver modules in each channel. So its not much above some of the older Sanyo/Fisher home gear of the 70's and 80's IMO...
The outputs are top notch, but the driver circuitry is text book Sanyo cause they use Sanyo driver modules. The power supply is very simple but the control chip is odd-ball, and can be difficult to locate. I found two in product samples from TI.
The frontend is also nothing special, just a array of op-amps like any other amp. NE5532 I think, its been a while so forgive me if I am incorrect about this part number ID. MAC did not use esoteric part anywhere inside this amp, so its no big deal to find everything they did use except the PWM control chip. In fact they could have used much stouter mosfets in the supply IMO.
Also look into cleaning and possibly soldering all the 12 volt feed wires inside as these are just using male and female slip connectors and they do fail with age carrying the amperage this amp can draw. Oh and the fans are also high on the list of must be replaced items. They are also nothing special and easily found locally to me he in Nor Cal.
You will find simple carbon film resistors used through out the amp and 5% tolerance is typical. I would have preferred metal film 2% but this was not my design.
I have seen the main caps in the supply leak and burn wholes into the amp board the size of a quarter or better so with any amp this old plan on a re-cap of the entire amp, and I suggest you use the very best you can find rated at no less then 105 C. I suggest you stick with name brand like Sanyo or Nichicon, nothing less.
The EQ units that were mounted on the top of the amp under the glass panels were also nothing too special and unless you have one of these I would not worry too much, I think the amps sounds better without all those IC's plugged into the circuitry. Plus I EQ separately using 31 band 1/3 octave anyway, so the MAC EQ was again a bit below my standards. But the remote programmer was cool if you like over-priced gadgets that are rare to even see nowadays at any price.
I loved working on mine except for the soft clip feedback to the power supply that in real life drops the rails so the amp has lower power output something like 90 watts or so instead of the 100 it is rated at. This always sounded like power compression to me no matter how it was explained to me, and this is dynamic range compression of the music your listening to. Similar to heavy volt amp current limiting that some amps use to protect themselves.
They do meet there advertised spec's, and compared to many car amps this is a big deal. And was one of the big selling points of MAC dealers. They sound OK. I just think they could sound better with better parts inside of them, especially for the money they charged for these back in the day. I own a MC-4000, and a MC-431 and if you a big MAC fan you know how much money those two amps sold for.
The problem your having is a bit unusual, and I would have to find the input electrolytic cap to each main amp channel and lift it out of circuit and separate the frontend from each main amp channel and test the main amp channels to see if they are the issue, or if it is the frontend. I have seen op-amps fail in the frontend so this is possible, and by splitting them up and running the amp directly inside you can rule out the main power amp channels.
Repairing this amp will be a process of deduction and logic as with all amps, so understanding the basic internal design structure is kind of important. Disconnecting sections is not too hard or time consuming and it will yield valuable guidance in your efforts.
I am bordering on info overload here
so I will shut down for the night and give you a chance to look your amp over and get back with me here. I will try my best to help guide you. If possible please post a pic of two of your amps internals if it is apart on your bench. Mine are stored in my garage somewhere, along with a couple hundred other car amps
Be careful the clamps are none to sturdy either. I never broke any of them but bending them can be easily done and then they have inadequate pressure to actually clamp the semiconductor to the sink properly. The sink is a monster and I loved that part of their design.
As for the protection circuitry well it uses direct feedback < I.E. non isolated > to the supply to soft clip the amp and here in lies the problem. If and when it does act up it causes the amp supply to do very odd things. Without a schematic bypassing is very troublesome and this is the only way I know to get the amp to turn on and be tested. Sort of a catch 22. MAC never was very open about this amp and of course PDF file sharing is something I never knew about from them even now..
The amp is really nothing special. it is based on Sanyo Fisher designs and has Sanyo channel driver modules in each channel. So its not much above some of the older Sanyo/Fisher home gear of the 70's and 80's IMO...
The outputs are top notch, but the driver circuitry is text book Sanyo cause they use Sanyo driver modules. The power supply is very simple but the control chip is odd-ball, and can be difficult to locate. I found two in product samples from TI.
The frontend is also nothing special, just a array of op-amps like any other amp. NE5532 I think, its been a while so forgive me if I am incorrect about this part number ID. MAC did not use esoteric part anywhere inside this amp, so its no big deal to find everything they did use except the PWM control chip. In fact they could have used much stouter mosfets in the supply IMO.
Also look into cleaning and possibly soldering all the 12 volt feed wires inside as these are just using male and female slip connectors and they do fail with age carrying the amperage this amp can draw. Oh and the fans are also high on the list of must be replaced items. They are also nothing special and easily found locally to me he in Nor Cal.
You will find simple carbon film resistors used through out the amp and 5% tolerance is typical. I would have preferred metal film 2% but this was not my design.
I have seen the main caps in the supply leak and burn wholes into the amp board the size of a quarter or better so with any amp this old plan on a re-cap of the entire amp, and I suggest you use the very best you can find rated at no less then 105 C. I suggest you stick with name brand like Sanyo or Nichicon, nothing less.
The EQ units that were mounted on the top of the amp under the glass panels were also nothing too special and unless you have one of these I would not worry too much, I think the amps sounds better without all those IC's plugged into the circuitry. Plus I EQ separately using 31 band 1/3 octave anyway, so the MAC EQ was again a bit below my standards. But the remote programmer was cool if you like over-priced gadgets that are rare to even see nowadays at any price.
I loved working on mine except for the soft clip feedback to the power supply that in real life drops the rails so the amp has lower power output something like 90 watts or so instead of the 100 it is rated at. This always sounded like power compression to me no matter how it was explained to me, and this is dynamic range compression of the music your listening to. Similar to heavy volt amp current limiting that some amps use to protect themselves.
They do meet there advertised spec's, and compared to many car amps this is a big deal. And was one of the big selling points of MAC dealers. They sound OK. I just think they could sound better with better parts inside of them, especially for the money they charged for these back in the day. I own a MC-4000, and a MC-431 and if you a big MAC fan you know how much money those two amps sold for.
The problem your having is a bit unusual, and I would have to find the input electrolytic cap to each main amp channel and lift it out of circuit and separate the frontend from each main amp channel and test the main amp channels to see if they are the issue, or if it is the frontend. I have seen op-amps fail in the frontend so this is possible, and by splitting them up and running the amp directly inside you can rule out the main power amp channels.
Repairing this amp will be a process of deduction and logic as with all amps, so understanding the basic internal design structure is kind of important. Disconnecting sections is not too hard or time consuming and it will yield valuable guidance in your efforts.
I am bordering on info overload here
so I will shut down for the night and give you a chance to look your amp over and get back with me here. I will try my best to help guide you. If possible please post a pic of two of your amps internals if it is apart on your bench. Mine are stored in my garage somewhere, along with a couple hundred other car ampsThanks again for your detail info. It's not overload at all. It's actually very useful. I'll probably start from replacing all the caps first. Then re-solder the power devices, then check the frontend, then the protection/soft clip circuitry. I'll see how things goes.
Now I have a quit question about my MC420M. I have replaced one of its 5532 op-amp's before (just like you said, a common failing part of the MAC's). Now it seems the output of the four channels are not balanced. The volume of 1 and 3 are louder than 2 and 4, even when turning all four volume nobs all the way down to the lowest. Have you seen this problem before on your MAC's? Any suggestion what might have gone wrong? Thanks a lot for your help!
Now I have a quit question about my MC420M. I have replaced one of its 5532 op-amp's before (just like you said, a common failing part of the MAC's). Now it seems the output of the four channels are not balanced. The volume of 1 and 3 are louder than 2 and 4, even when turning all four volume nobs all the way down to the lowest. Have you seen this problem before on your MAC's? Any suggestion what might have gone wrong? Thanks a lot for your help!
Mis-matched gain after op-amp replacement hummm...This could be caused by several reasons.
Dissimilar op-amp die manufacturing. Process's vary from maker to maker. So matching correct manufacturer parts can be an issue. The circuit may be part sensitive enough that not using the exact part could cause a gain loss..< Big maybe here >
Damage may have occurred to one of the other op-amps stages and then both would have needed to be replaced. < very possible, they are so cheap I would gut and start over with matching brand >
If it was the gain IC then the gain pot may have been damaged < seen this one before myself > And if there were any caps in the feedback then these could be partially shorted or leaky and this could cause gain related issues also. This might have been why it originally failed also.
A big lot of if's and what abouts there, but its the best I can muster without actually seeing the amp in person.
A simple test is to use a mono Y adapter and send a clean sine wave to both channels and then O-scope both till you find the drop out. Of course you would need to set the gains and what not at the same levels to do this testing. The amp would not need to be loaded with speaker just a clean signal in and a good scope to view each matching legs signal and then just A B each channel till the loss is found. A bit of a time sink but not too draining...
Oh if I sound overly critical of MAC, please understand that I have seen and worked on most everybody's amp out there and I was kind of expecting more when I opened my first MAC up to repair on my bench. I mean look at their reputation and what not and you just get the idea that they would at least be able to design their on amp channel without using Sanyo driver modules. Just about everybody else does their own design using raw parts, not hybrid IC modules.
The only other amps I saw using these were very early ADS, SANYO, and Pyramid amps. I just expected much higher standards from MAC..... and I apologize to any and all MAC lovers out there just in case I might be offending anyone's sensibilities...
Dissimilar op-amp die manufacturing. Process's vary from maker to maker. So matching correct manufacturer parts can be an issue. The circuit may be part sensitive enough that not using the exact part could cause a gain loss..< Big maybe here >
Damage may have occurred to one of the other op-amps stages and then both would have needed to be replaced. < very possible, they are so cheap I would gut and start over with matching brand >
If it was the gain IC then the gain pot may have been damaged < seen this one before myself > And if there were any caps in the feedback then these could be partially shorted or leaky and this could cause gain related issues also. This might have been why it originally failed also.
A big lot of if's and what abouts there, but its the best I can muster without actually seeing the amp in person.
A simple test is to use a mono Y adapter and send a clean sine wave to both channels and then O-scope both till you find the drop out. Of course you would need to set the gains and what not at the same levels to do this testing. The amp would not need to be loaded with speaker just a clean signal in and a good scope to view each matching legs signal and then just A B each channel till the loss is found. A bit of a time sink but not too draining...
Oh if I sound overly critical of MAC, please understand that I have seen and worked on most everybody's amp out there and I was kind of expecting more when I opened my first MAC up to repair on my bench. I mean look at their reputation and what not and you just get the idea that they would at least be able to design their on amp channel without using Sanyo driver modules. Just about everybody else does their own design using raw parts, not hybrid IC modules.
The only other amps I saw using these were very early ADS, SANYO, and Pyramid amps. I just expected much higher standards from MAC.....
and I apologize to any and all MAC lovers out there just in case I might be offending anyone's sensibilities...Thanks again for your help. I think I have confused you a little bit here. The mismatched gain didn't happen right after I replaced the 5532. It had been working fine for at least a year or so after that. Then I bought my 431M to replace my 420M. And the 420M has been sitting my storage room since then. I only found this problem couple months ago when a friend of mine borrowed it for testing.
Anyway, I'll take a look into it later. Right now I wana focus on my 431M first. I just replaced couple more bad caps (also the 50v 1uF ones) in the meter panel. Now it seems the first 2 channels are working fine and the last 2 channels still have problem. It seems much more clear now that it's the bad caps causing the problem indeed.
I found that the 3-4 channels meter behave like a capacitor reading now. When I first turn on the amp (without input or output), the 3-4ch meter still shoots over the scale, but it slowly comes back down and eventually reaches 0 (just like testing a capacitor with an analog multimeter). After it reaches 0 for a while, if I turn it off then on again right away, then it'll work normally. But if I turn it off for a while then turn it back on, the 3-4ch meter will jump then slowly come back to 0. The longer the amp stays off, the higher the meter will jump to when I turn it back on.
Now I'll definitely replace all the caps. But I couldn't find all the caps in the local shops. They don't carry any good ones anyway. So I'll have to get them online. But meanwhile I'll try replacing the ones I can find locally first and see if it will solve the problem or not. I believe it's the caps in the input/output portions are at fault to me, because that's what the meter is reading, right? Thanks!
Anyway, I'll take a look into it later. Right now I wana focus on my 431M first. I just replaced couple more bad caps (also the 50v 1uF ones) in the meter panel. Now it seems the first 2 channels are working fine and the last 2 channels still have problem. It seems much more clear now that it's the bad caps causing the problem indeed.
I found that the 3-4 channels meter behave like a capacitor reading now. When I first turn on the amp (without input or output), the 3-4ch meter still shoots over the scale, but it slowly comes back down and eventually reaches 0 (just like testing a capacitor with an analog multimeter). After it reaches 0 for a while, if I turn it off then on again right away, then it'll work normally. But if I turn it off for a while then turn it back on, the 3-4ch meter will jump then slowly come back to 0. The longer the amp stays off, the higher the meter will jump to when I turn it back on.
Now I'll definitely replace all the caps. But I couldn't find all the caps in the local shops. They don't carry any good ones anyway. So I'll have to get them online. But meanwhile I'll try replacing the ones I can find locally first and see if it will solve the problem or not. I believe it's the caps in the input/output portions are at fault to me, because that's what the meter is reading, right? Thanks!
Hi 1moreamp, I need a little more help here. Please give me some advice.
I replaced all the caps on my 431M already. But somehow it got worse. Now it won't turn on at all (stays in protection mode). The problem still behaves somewhat like a cap tho: When I tried to turn it on the first time after replacing the caps, the 3-4ch meter jumped over the scale then quickly came back down to zero (the 1-2ch meter just flexed a little). And the power guard light stayed on at all time. If I try to turn it on again right away (disconnect "ON" and reconnect it again immediately), the 3-4ch meter will jump to a lower mark. The longer I keep it "ON" before doing the re-turn-on, the lower the meter will jump. If I let it stay off for a while before I try to turn it on again, then the meter will jump back to a higher mark (even over the scale if I let it stay off long enough, just like I tried to turn it on the first time).
Then I replaced the 5532 op-amps. But it still behaves the same. So I tracked down the last two op-amps (45580's) in the front end section. Then I lifted all 4 resisters connecting to outputs of these two 45580's. This should separate the front end from the main amp sections, right? It still gave me the same problem. So does it mean that the main amp section is bad here? I tested all the MOSFET's, they all seem fine tho.
Now what should I do next? Any suggestion? Thanks alot!
BTW, I fixed my 420M already. It was the bad gain pots. I cleaned them with some WD-40 then problem solved. I also replaced the caps and the 5532's just in case.
I replaced all the caps on my 431M already. But somehow it got worse. Now it won't turn on at all (stays in protection mode). The problem still behaves somewhat like a cap tho: When I tried to turn it on the first time after replacing the caps, the 3-4ch meter jumped over the scale then quickly came back down to zero (the 1-2ch meter just flexed a little). And the power guard light stayed on at all time. If I try to turn it on again right away (disconnect "ON" and reconnect it again immediately), the 3-4ch meter will jump to a lower mark. The longer I keep it "ON" before doing the re-turn-on, the lower the meter will jump. If I let it stay off for a while before I try to turn it on again, then the meter will jump back to a higher mark (even over the scale if I let it stay off long enough, just like I tried to turn it on the first time).
Then I replaced the 5532 op-amps. But it still behaves the same. So I tracked down the last two op-amps (45580's) in the front end section. Then I lifted all 4 resisters connecting to outputs of these two 45580's. This should separate the front end from the main amp sections, right? It still gave me the same problem. So does it mean that the main amp section is bad here? I tested all the MOSFET's, they all seem fine tho.
Now what should I do next? Any suggestion? Thanks alot!
BTW, I fixed my 420M already. It was the bad gain pots. I cleaned them with some WD-40 then problem solved. I also replaced the caps and the 5532's just in case.
Well lets digress a bit. The meters jumping bothers me. My 431 lacks meters but the 4000 does and I do no recall ever seeing them peg full scale on turn on.
This could indicate a huge surge in the channel and such things will shutdown the amp as I recall the power guard circuitry will detect this as a overload condition and ramp down the supply < or shut it off entirely >.
If you have isolated the front end and the problem still exists them the problem is most certainly in your main amp sections. Both meters pegging may mean nothing depending on where they take their signal from Also look for any DC or pulsed output on each channel.
These amps use double sided circuit boards and are you sure the connection from top to bottom were not broken when you re-capped the amp < I see this problem a lot on some PC boards of other big brand amps out there. If there is a break and the amp channels do not see both rails evenly applied this could be also causing similar issues.
I don't want to come off sounding antagonistic but I really need you to recheck all the solder joints on all the caps you replaced. If the solder failed to flow to both sides and connect them then this could just aggravate the problems you already had. I ask this in hind site of my own efforts over the years where all my efforts back fired on me and caused me to rework my own work till I found what I have done incorrectly ....Yes i make mistakes even with 30 years into this stuff
PM me if you like on this matter, we may need to collaborate more closely then possible on the open forum....
This could indicate a huge surge in the channel and such things will shutdown the amp as I recall the power guard circuitry will detect this as a overload condition and ramp down the supply < or shut it off entirely >.
If you have isolated the front end and the problem still exists them the problem is most certainly in your main amp sections. Both meters pegging may mean nothing depending on where they take their signal from Also look for any DC or pulsed output on each channel.
These amps use double sided circuit boards and are you sure the connection from top to bottom were not broken when you re-capped the amp < I see this problem a lot on some PC boards of other big brand amps out there. If there is a break and the amp channels do not see both rails evenly applied this could be also causing similar issues.
I don't want to come off sounding antagonistic but I really need you to recheck all the solder joints on all the caps you replaced. If the solder failed to flow to both sides and connect them then this could just aggravate the problems you already had. I ask this in hind site of my own efforts over the years where all my efforts back fired on me and caused me to rework my own work till I found what I have done incorrectly ....Yes i make mistakes even with 30 years into this stuff
PM me if you like on this matter, we may need to collaborate more closely then possible on the open forum....
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